The invention concerns antennas, specifically small stacked patch antennas.
The size of mobile wireless terminals is decreasing as digital and analog components become increasingly integrated and miniaturized. Apart from user interface aspects, the main limiting factor on further size reductions are the antennas. The antennas are now a dominating factor in the visual appearance of many mobile devices. From an esthetic point of view it would be desirable to have antennas that are small. Further, manufacturing costs can usually be reduced with smaller antennas.
Wireless local-area network (LAN) solutions for office use are rapidly becoming a prominent competitor to traditional wireline networks. A major advantage of wireless LANs is the mobility they offer. A computer can be connected to a wireless LAN from anywhere within the LAN""s coverage area. The antennas for the mobile terminals of the wireless LANs are normally intended for installation on a PC-card, which puts constraints on the allowable antenna size. However, the dimensions of antennas are wavelength dependent. Additionally an antenna""s bandwidth and radiation efficiency are limited by the effective volume, in terms of wavelengths, that the antenna occupies.
Another constraint put on antennas is their radiation pattern. Wireless LAN antennas mounted on, for example, a PC-card should be small and radiate primarily in the horizontal plane. Indoor wave propagation tends to be confined to incidence angles within a narrow angular interval centered around the horizon. The antenna should also have an omni-directional radiation pattern, i.e. the radiation pattern should be substantially independent of the azimuthal angle, in order to be able to register the various wave components of a typical multipath propagation channel common in indoor environments. Thus, a wireless LAN antenna should be wideband, efficient and substantially omni-directional. Further, such an antenna should make an optimum use of its volume in order to fit into an alloted space in a respective device. Wireless LAN antennas intended to be mounted on a PC-card (direction of mounting in relation to computer orientation when in use should be taken into account), should therefore be planar and low-profile with a negligible thickness.
Additionally a wireless LAN antenna for indoor use should, apart from an omnidirectional radiation pattern with an essentailly constant radiation pattern in the azimuthal (horizontal) direction, preferably also have a null-depth, or a near null-depth, in the broadside (vertical) direction. A null-depth, or near null-depth in the broadside direction is important to enable different wireless LANs on different floors to co-exist with as little cross interference as possible.
A variety of small low profile antennas have been proposed. Examples include everything from antennas based on modifications of the traditional monopole antenna to elaborate optimized antenna schemes involving multi-layered structures with meandering lines, ceramic materials, and various types of impedence matching schemes. Most types of low profile antennas with wide bandwidths have semi-isotropic radiation patterns with maximum radiation, or at least significant radiation levels, in the broadside, i.e. vertical, direction. One type of antenna that addresses some of the above mentioned constraints is the bent stacked slot antenna (BSSA). The BSSA antenna achieves a relatively wide bandwidth and small size and makes use of a center strip of a middle patch as an integrated impedance matching unit. An example of such an antenna is described in the European patent application EP 795926. However, a disadvantage with the BSSA type of antenna can in some applications be considered to be the inherent azimuthal gain variations and relatively narrow bandwidth, i.e. there is a need for a more omni-directional antenna with a wider bandwidth.
An object of the invention is to define a low-profile antenna which provides a high efficiency, good omni-directionality and a wide bandwidth.
Another object of the invention is to define a low-cost low profile antenna which is suitable to be mounted on a PC-card.
A further object of the invention is to define a low profile antenna which when mounted horizontally provides a substantially omni-directional radiation pattern in the azimuthal direction and at least a near null-depth in the broadside direction.
The aforementioned objects are achieved according to the invention by a stacked patch antenna. The stacked patch antenna is intended to be mounted on a ground plane. The antenna comprises two stacked metallic patches. The patches are stacked on top of each other. The patch to be mounted closest to the ground plane, the middle patch, comprises at least two conductors at or close to its edge which conductors are intended to be connected to the ground plane to thereby ground the patch in two zero potential areas. The patch to be mounted furthest away from the ground plane, the top patch, comprises at least two conductors at: or close to its edge which electrically interconnect the two patches. The conductors electrically interconnecting the patches should preferably be connected to the middle patch at least proximate the respective zero potential areas of the middle patch. The conductors preferably also provide structural strength to the antenna and provide mounting means and support for the patches. The middle patch is fed at a feed area which is at least proximate the geometric center of the middle patch. The middle patch further comprises at least two apertures completely within the circumference of the middle patch. The apertures do not divide the middle patch into two or more physically and/or electrically separated parts, i.e. the middle patch is in one piece. Preferably the apertures are placed in such a way that at least two paths are provided from each place which is grounded on the middle patch to the feed area, i.e. each aperture blocks a direct line from the feed area to a respective place which is grounded. There is always at least one physical/electrical connection between the feed area and each zero potential area of the middle patch. Thereby enabling radiation from a slot defined by the edge of the top patch and the edge of the middle patch and a slot defined by the edge of the middle patch and the ground plane.
The aforementioned objects are also achieved according to the invention by a stacked patch antenna comprising two metallic patches stacked on top of each other. The middle patch comprises at least two conductors at or close to its edge, which conductors are intended to be connected to a ground plane to thereby ground the patch in two places. The top patch comprises at least two conductors at or close to its edge which electrically interconnect the two patches. The middle patch is fed at a feed area which is at least proximate Its geometric center. The middle patch further comprises at least two apertures completely within its circumference, i.e. each aperture having a respective unbroken circumference. Thereby enabling radiation from slots defined by the edge of the top patch and the edge of the middle patch and defined by the edge of the middle patch and the ground plane.
The aforementioned objects are also achieved according to the invention by a low profile antenna structure. The antenna structure comprises a first metallic patch and a second metallic patch stacked over a ground plane. The first patch comprises a circumference along a patch edge of the first patch. The second patch comprises a circumference along a patch edge of the second patch. The first patch is arranged between the ground plane and the second patch. The first patch is grounded at at least a first zero potential area by electrical connection with the ground plane and a second zero potential area by electrical connection with the ground plane. The first patch is further fed at a single feed area. The second patch is electrically interconnected to the first patch. According to the invention the first patch comprises at least a first aperture and a second aperture located completely within the circumference of the first patch, i.e a current can flow on the first patch completely around each aperture and a current can flow on the first patch from the feed area to each zero potential area. The presence of the apertures force current, propagating from the feed area to the first zero potential area and the second zero potential area, toward the patch edge of the first patch. By forcing the current to flow close to the edge there can be radiation from slots defined by the edge of the first patch and the edge of the second patch and the ground plane. The slots go around the antenna almost completely and therefore a substantially omni-directional radiation pattern is provided.
The aforementioned objects are also achieved according to the invention by a low profile antenna structure. The antenna structure comprises a first metallic patch and a second metallic patch stacked over the first patch. The patches are intended to be mounted over a ground plane. The first patch comprises a circumference along a patch edge of the first patch. The second patch comprises a circumference along a patch edge of the second patch. The first patch is arranged between the ground plane and the second patch. The first patch comprises a first zero potential area by connection with the ground plane and a second zero potential area by connection with the ground plane. The second patch is electrically interconnected to the first patch. The antenna is fed at a single feed area comprised on the first patch. According to the invention the first patch comprises at least a first aperture and a second aperture located completely within the circumference of the first patch, i.e. the first patch comprises two apertures with edges that do not even touch the edge of the first patch. By providing these apertures, current, propagating from the feed area to the first zero potential area and the second zero potential area, is forced toward the patch edge of the first patch to. By forcing the current to take these paths radiation is enabled from slots defined by the edge of the first patch and the edge of the second patch and the ground plane.
Advantageously the first aperture and the second aperture are located on the first patch in such a way that current propagating from the feed area to the first zero potential area propagates in two different paths around the first aperture and that current propagating from the feed area to the second zero potential area propagates in two different paths around the second aperture. Preferably the first aperture is located between the feed area and the first zero potential area, and the second aperture is preferably located between the feed area and the second zero potential area. Advantageously the second patch is electrically interconnected to the first patch at at least the first zero potential area and the second zero potential area.
Preferably, to ensure that the current propagates where desired, the first aperture and the second aperture each have an extension which is substantially perpendicular to a line between the first zero potential area and the second zero potential area, i.e. the apertures are longer than they are wide
In certain embodiments there is a symmetry of the first patch about a line between the first zero potential area and the second zero potential area. In other embodiments, alone or in combination, there is a symmetry of the first patch about a line perpendicular to a line between the first zero potential area and the second zero potential area. Other embodiments are more or less asymmetric.
In some embodiments the second patch comprises no openings within its circumference. In other embodiments the second patch comprises at least one opening within its circumference. In further embodiments the second patch is electrically split into two halves along a line which is substantially perpendicular to a line between the first zero potential area and the second zero potential area.
Preferably the second patch at least covers the first aperture and the second aperture of the first patch.
In some embodiments the first patch comprises further apertures. In some embodiments the antenna structure comprises the ground plane. Then, advantageously the ground plane is substantially of the same size as the first patch and the second patch. In some embodiments the first patch and the second patch are substantially of the same size. In certain applications the first patch, in addition to the first aperture and the second aperture, advantageously comprises further apertures.
In some embodiments the electrical connections from the first patch to the ground plane and the electrical interconnections between the first patch and the second patch, in addition to providing the antenna structure with electrical connections also provides the antenna with mechanical support giving the antenna a self supporting structure. In other embodiments the first patch is supported by a first dielectric and the second patch is supported by a second dielectric, the first dielectric and the second dielectric further providing the antenna with mechanical support giving the antenna a self supporting structure. In other embodiments comprising the ground plane it can be advantageous that the first patch is supported by a first dielectric and that the second patch is between the first dielectric and a second dielectric and that the ground plane is supported by the second dielectric, the first dielectric and the second dielectric further providing the antenna with mechanical support giving the antenna a self supporting structure.
The antenna structure according to the invention may at the single feed area be probe fed at one point, thereby attaining a shielded feed probe. The single feed area can then also further comprise inductive feed matching. Optionally the antenna structure may at the single feed area be fed by an aperture coupling. Alternatively the single feed area may be probe fed at a plurality of points. The plurality of points can advantageously be placed in the feed area along a limited line that if extended would pass through the first zero potential area and the second zero potential area. Preferably the plurality of points are placed in the feed area symmetrically about a line that passes through the first zero potential area and the second zero potential area.
The different additional enhancements of the antenna structure according to the invention can be combined in any desired manner as long as no conflicting features are combined.
The aforementioned objects are also achieved according to the invention by a device comprising wireless communication means, which device comprises an antenna according to any above described antenna structure according to the invention.
The aforementioned objects are also achieved according to the invention by a wireless or wireless mobile terminal which comprises an antenna according to any above described antenna structure according to the invention for wireless communication.
The aforementioned objects are also achieved according to the invention by a personal computer card suitable for insertion into an electronic device, which card comprises an antenna according to any above described antenna structure according to the invention
The aforementioned objects are also achieved according to the invention by a wireless local area network system comprising a base station and a plurality of terminals which are in wireless communication with the base station, where at least one terminal comprises either directly, i.e. permanently mounted in the terminal, or indirectly, i.e. removably mounted in the wireless terminal, an antenna according to any above described antenna structure according to the invention.
By providing a low-profile stacked patch antenna according to the invention a plurality of advantages over prior art antennas are obtained. Primary purposes of the invention are to provide a substantially omni-directional antenna with a low-profile that is suitable for mounting on a PC-card, which is still efficient and has a wide bandwidth, for use in a, for example, wireless LAN. Other advantages of this invention will become apparent from the detailed description.